AddressableBlock.php

<?php

namespace Drupal\block\Controller;

use Symfony\Component\HttpFoundation\Response;

class AddressableBlock {
  public function respond($block_id, $route) {
    $build = array();
    $entity = block_load($block_id);
    $build[$block_id] = $entity->getPlugin()->build();
    $build[$block_id]['#block'] = $entity;
    return new Response(drupal_render($build));
  }
}